Multiple library folder locations?

Ok so I have known about the OSX Library folders for years. But recently I needed to do a complete wipe and install of Sierra, and upon setting up my backup selection again in Arq 5 I noticed something that I don't recall having been aware of in the past. I knew there was a System Library folder and a User Library folder, but now I see the count is actually 3 folders, not two!? Trying to search for "How many library locations does OSX have?" and similar queries only returns how-to's to navigate and show them, not what I was looking for.

I have Library folders in these locations:

1. MacHD root /
2. MacHD system folder MacHD/System/
3. Home folder MacHD/Users/Satori/

Can someone confirm whether this is correct or if something got wacky over the years?